12c Black (17). Large
margins to in, used with vertical pair and single of 3c Dull Red (11),
margins to in, oxidized, tied or cancelled by grids on 1855 folded cover
to Havre, France, red "New Orleans La. Apr." datestamp, "New York
Am. Packet Apr. 18" datestamp, Calais transit ties 3c pair, French "8"
decimes due marking, receiving backstamp, some toning, otherwise Fine cover
paying the 21c rate via American Packet
(Image) 
12c Black (17).
Horizontal pair, large margins to just in at top left, tied by blue grids,
matching "Skaneatles N.Y. Aug. 24" datestamp on 1857 cover to Bristol,
England, red "19" credit handstamp, Boston transit and receiving
backstamps, pair lifted and replaced, appears fresh and Very Fine, late
usage of the imperforate 12c, with 1978 P.F. certificate (Image) 
12c Black (17). Three
large margins, in at top, used with 1857 12c Black (36) and
cancelled by Boston large "Paid" grid on small Sept. 1857 cover to
London, England, red credit and transit markings, Very Fine and rare,
only two such combinations are recorded, ex Grunin
(Image) 
12c Black (17). Used with
Hawaii, 1857, 5c Blue on Thin White Wove (8), both stamps margins to
in and tied by "San Francisco Cal. Jul. 16, 1860" datestamp on buff cover
to Fall River Mass., red "Honolulu U.S. Postage Paid Jun. 23" originating
datestamp, slightly reduced at left, Very Fine and attractive usage (Image) 
12c Black (17).
Horizontal pair, ample to large margins, tied by bold grids, matching
"Brooklyn N.Y. Jul. 31" datestamp on 1857 cover to London, England and
forwarded to Matlock, England, red "3" credit, red transit also ties
pair, Great Britain 1856 1p Red, Plate 14 (20) affixed over pair and
tied by "26" numeral cancel to pay forwarding postage, blue oval "Forwarded
by Geo. Peabody & Co. London" backstamp, minor edge wear and slightly
reduced at left, otherwise Very Fine and very rare combination usage (Image) 
12c Black (17).
Horizontal pair, huge margins showing bits of surrounding stamps, tiny
margin tear at left, discreet ms. check mark on one, tied by blue
"Cincinnati O. Aug. 17" circular datestamp on 1857 folded letter to
Liverpool, England, and forwarded to Milan, Italy, with Great Britain 1856
6p Lilac, 1sh Green (27, 28), tied by Liverpool "spoon" duplex, lengthy
sequence of transits including red "19" credit handstamp and red Liverpool
receiving cds, followed by Brown Shipley & Co. forwarder's oval, red "PD"
in circle and black Calais transit cds, appropriate
backstampsEXTREMELY FINE. A COLORFUL AND EXTREMELY RARE COMBINATION OF UNITED STATES IMPERFORATE AND GREAT BRITAIN PERFORATED CLASSIC ISSUES.
Approximately one month prior to the mailing date of this cover, the first United States stamps with perforations were issued. The Bemrose machine used to perforate Great Britain's stamps was the model for the Toppan, Carpenter perforator. On this cover, we have the old and the new together at the time of transition. Ex Sevenoaks (Image)
12c Black, Diagonal Half Used
as 6c (17a). Lower left half, tied by centrally-struck "San Francisco
Cal. Jul. 16 Paid 6" datestamp on blue 1853 folded cover to Boston,
endorsed "Via Panama/15 July 53", faint vertical file fold clear of
stamp and markingsEXTREMELY FINE GEM. AN OUTSTANDING BISECT OF THE 12-CENT 1851 ISSUE IN THE FINEST ATTAINABLE CONDITION. A WONDERFUL COVER.
Ex Caspary (Image)
12c Black, Diagonal Half Used
as 6c (17a). Top right half with huge sheet margin, used with 3c
Brownish Carmine (11), large margins to barely in, bleached spot at
upper right, tied by two strikes of "U.S. Express Mail N.Y./N. York/May
15" route agent's circular datestamp on 1852 blue folded letter to
Providence R.I., file folds clear of stamps, a few letters of addressee's
last name changedA FINE AND EXTREMELY RARE 12-CENT BISECT USED IN COMBINATION WITH 3-CENT 1851 TO MAKE UP TRIPLE RATE. ONE OF THE FEW KNOWN 12-CENT 1851 BISECTS USED ON INTRA-EAST COAST MAIL.
The interior portion of this letter is addressed to "Bailey & Gallup" as written on the address panel, therefore, it appears that the sender made the change before mailing.
Ex Grunin, Sevenoaks. With 1975 P.F. certificate (Image)